1. Field of the Invention
The present invention relates to linear motors used in industrial machines such as machine tools.
2. Description of the Related Art
Linear motors have conventionally been used in industrial machines such as machine tools for realizing high-speed and high-accuracy. Among such linear motors, there are some that have realized low cost particularly in long-stroked machines by disposing expensive permanent magnets on the mover side and thus allowing the use of fewer permanent magnets. (For example, see Japanese Patent Laid-Open Publication No. 2007-318839 below.) An example of a conventional linear motor will be explained with reference to FIGS. 7 through 9. FIG. 7A is a diagram showing a schematic structure of a conventional linear motor, and FIG. 7B and C show arrangements of permanent magnets. FIG. 8 is a sectional diagram of the linear motor in FIG. 7A taken along a line C-C. FIG. 9 is a connecting diagram of coils wound in a linear motor.
A linear motor has two stators 52a, 52b extending in parallel and a mover 51 movable between the stators 52a, 52b along a direction in which the stators 52a, 52b extend.
The stators 52a, 52b are formed by laminating magnetic steel sheets. The stators 52a, 52b have salient poles 50 on surfaces opposing each other at a predetermined pitch, at pitch P, for example. Further, the stators 52a, 52b are prepared in a predetermined length Las shown in FIG. 7A. A plurality of stators 52a, 52b are disposed along the stroke of the mover 51 in a traveling direction of the mover 51. The stators 52a, 52b are fixed, for example, on a base 72 of a machine tool (shown in FIG. 8). Specifically, as shown in FIG. 8, the stators 52a, 52b are fixed by a bolt 71 such that a bottom face 74 of the stator contacts the base 72.
On the other hand, the mover 51 is movably supported in the X-axis direction in FIG. 7 by a rolling guide or the like provided between the base 72 and a table (now shown) and fixed to the table. The mover 51 has mover blocks 53, 54, 55 formed by laminating magnetic steel sheets. The mover block 53 is a mover block for the U-phase, the mover block 54 is a mover block for the W-phase, and the mover block 55 is a mover block for the V-phase. The mover blocks 53, 54, 55 are arranged such that they are relatively displaced by 120°, that is, by one third of the pole pitch P of the stators 52a, 52b, in the X-axis direction which is the direction of travel of the mover 51. A part of the mover blocks 53, 54, 55 are in some cases mechanically connected to each other in order to maintain dimensional accuracy between the blocks.
Three-phase alternating current coils are wound around each of the mover blocks 53, 54, 55. That is, a three-phase alternating current coil 56 for the U-phase is wound around the mover block 53, a three-phase alternating current coil 57 for the W-phase is wound around the mover block 54, and a three-phase alternating current coil 58 for the V-phase is wound around the mover block 55, respectively. The mover blocks 53, 54, 55 around which the three-phase alternating current coils 56, 57, 58 are wound are integrally formed by a mold resin 76.
Permanent magnets 59, 64 are arranged on the surface of the mover blocks 53, 54, 55 such that N and S poles alternate. Specifically, as shown in FIGS. 7B, C, three pairs of permanent magnets, a pair comprising an N and an S, are arranged at a pitch P. Here, as shown in FIG. 7A, supposing that the stator 52a side is SIDE-A and the stator 52b side is SIDE-B, the permanent magnets 59 on the SIDE-A and the permanent magnets 64 on the SIDE-B are arranged such that the polarity as seen from the SIDE-A is opposite to the polarity as seen from the SIDE-B.
The three-phase alternating coils 56, 57, 58 are connected in a star connection as shown in FIG. 9. As shown in FIG. 7A, for example, when a current is applied to the three-phase alternating current coils 56, 57, 58 from U in the directions of V and W, a magnetic flux 62 is excited in the linear motor.
Now, the operation of the linear motor will be described. When current is applied to the three-phase alternating current coils 56, 57, 58, the mover blocks 53, 54, 55 are excited in the positive direction or in the negative direction on the Y-axis (refer to FIG. 7A). At that time, out of the permanent magnets 59, 64, magnetic flux of the permanent magnets arranged in the same magnetization direction as the direction in which the alternating current coils is excited will be strengthened. On the other hand, magnetic flux of the permanent magnets arranged in the opposite direction of the direction in which the alternating coils is excited will be weakened. Accordingly, the permanent magnets 59 and 64 will be excited such that the polarities will be opposite to each other, that is, one will serve as the N pole and the other will serve as the S pole. Magnetic fluxes having passed through the respective mover blocks 53, 54, 55 and the stator 52a, 52b sides form a flux path as shown by reference numeral 62 in FIG. 7A. At this time, magnetic attractive force is generated depending on the positions of the mover 51 and the stators 52a, 52b, generating thrust in the mover 51, resulting in a movement of the mover 51.
The magnetic flux flow will now be explained in further detail. Suppose that current is directed from the U-phase to the V and W-phases, that is, in the winding direction shown in FIG. 7A in the case of three-phase alternating current coil 56 and in the opposite direction of the winding direction shown in FIG. 7A in the case of three-phase alternating current coils 57, 58. As a result, the SIDE-A becomes the S-pole and the SIDE-B becomes the N-pole in the case of the mover block 53. In contrast, in the case of mover blocks 54, 55, the SIDE-A becomes the N-pole and the SIDE-B becomes the S-pole. Consequently, as shown in FIG. 7A, a magnetic path 62 is formed such that the magnetic flux from the mover block 53 passes through the stator 52b to the mover blocks 54, 55, then through the stator 52a and back to the mover block 53. As a result, magnetic attractive force in the X-axis direction acts on the mover 51 and therefore thrust is generated.